The overall goal of the proposed research is to reveal the underlying microstructure of verbal memory retrieval deficits in aphasia through a detailed analysis of the temporal output pattern of free-recall responses in early and later stages of learning. The ability to acquire and retrieve new information is essential to all other aspects of behavior and a better understanding of the mechanisms underlying the breakdowns in aphasia would have particular relevance with regard to aphasia therapy and recovery from aphasia. Five experiments are proposed that are designed to investigate systematically the contributions of organizational and semantic factors to aphasic retrieval deficits. Emphasis will be placed on the contributions of these factors to retrieval deficits in anterior versus posterior aphasic syndromes. Identification of the components of the retrieval failures that are specifically related to language disturbances associated with anterior and posterior aphasic syndromes should be possible from a detailed analysis of the temporal dynamics of free-recall. The research question of particular interest is the extent to which (1) retrieval dynamics of aphasic patients with anterior lesions may be associated with deficits in organizational abilities (executive systems) and (2) the retrieval patterns associated with posterior aphasic patients will be associated with deficits of semantic systems. All experiments will include four subject groups; anterior aphasics, posterior aphasics, left-hemisphere damaged non-aphasic patients, and age-matched normal controls. Participants will be presented with study lists consisting of objects or words that can either be organized into different semantic categories or are unrelated. Study lists will be presented to each subject over multiple successive trials with free-recall following each one. Following the completion of the learning and recall trials, retention of the list items will also be assessed using a recognition format.